1. Field of the Invention
The present invention relates to an electron beam apparatus and an image producer as an application thereof, such as an image display and the like. The present invention also relates to a spacer for use in the electron beam apparatus.
2. Related Background Art
There are two types of electron emission devices currently known: a hot cathode element and a cold cathode element. As to the latter, the known elements include, for example, surface conduction type electron emission devices, field emission elements (hereinafter referred to as an FE type) and metal-insulating layer-metal type electron emission devices (hereinafter referred to as an MIM type).
The surface conduction type electron emission devices currently known include, for example, one disclosed by M. I. Elinson in Radio Eng. Electron Phys., 10, 1290, (1965), and the others described below.
The surface conduction type electron emission devices take advantage of the phenomenon that electron emission occurs on the thin film of a small area formed on the substrate when applying electric current parallel to the surface of the film. There are several types of surface conduction type electron emission devices reported, in addition to the aforesaid element by Elinson et al. which utilizes SnO2 thin film: one utilizing Au thin film (refer to G. Dittmer: “Thin Solid Films,” 9, 317 (1972)), one utilizing In2O3/SnO2 thin film (refer to M. Hartwell and C. G. Fonstad: “IEEE Trans. ED Conf.,” 519 (1975)), and one utilizing carbon thin film (refer to Hisashi Araki et al. “Vacuum,” Vol. 26, No. 1, 22 (1983)).
FIG. 23 shows a plan view of the aforementioned element by M. Hartwell et al. as a typical example illustrating the construction of the surface conduction type electron emission devices. In the figure, reference numeral 3001 designates a substrate and numeral 3004 designates a conductive thin film consisting of metal oxide and formed by sputtering. The conductive thin film 3004 is in the form of an H-shaped plan as shown in the figure. An electron emission portion 3005 is formed by conducting an energization treatment, known as energization forming which is to be described below, to the above conductive thin film 3004. The spacings L and W in the figure are set for 0.5 to 1 [mm] and 0.1 [mm], respectively. For convenience's sake, in the above figure the electron emission portion 3005 is shown in the center of the conductive thin film 3004 in the form of a rectangle. The figure is, however, very schematic and does not necessarily represent the actual position and form of the electron emission portion.
In the aforesaid surface conduction type electron emission devices, including one by M. Hartwell, it has been common that the electron emission portion 3005 is formed by conducting an energization treatment, called energization forming, to the conductive thin film 3004 prior to the execution of electron emission. Energization forming used herein means that a constant direct-current voltage or a direct-current voltage stepping up at a very slow rate of, for example, about 1 V/min is applied to both ends of the conductive thin film 3004 to pass a current therethrough and cause a local fracture, deformation or change in quality therein, so as to form the electron emission portion 3005 in a highly resistive state. In some part of the conductive thin film 3004 having undergone a local fracture, deformation or change in quality, a crack is to appear. When applying a proper voltage to the conductive thin film 3004 after the above energization forming, electric emission occurs in the vicinity of the above crack.
The known FE type elements include, for example, one disclosed by W. P. Dyke & W. W. Dolan in “Field Emission,” Advance in Electron Physics, 8, 89 (1956) and one disclosed by C. A. Spindt in “Physical Properties of Thin-Film Field Emission Cathodes with Molybdenium cones,” J. Appl. Phys., 47, 5248 (1976).
FIG. 25 shows a sectional view of the aforementioned element by C. A. Spindt et al. as a typical example illustrating the configuration of FE type elements. In the figure, reference numeral 3010 designates a substrate, numeral 3011 an emitter wiring consisting of a conductive material, numeral 3012 an emitter cone, numeral 3013 an insulating layer and numeral 3014 a gate electrode. In this element, field emission is caused at the tip portion of the emitter cone 3012 by applying a proper voltage between the emitter cone 3012 and the gate electrode 3014.
There is another example of the construction of FE type elements where, unlike the laminated structure shown in FIG. 25, an emitter and a gate electrode are arranged on the substrate almost parallel to the substrate plane.
The known MIM type elements include, for example, one disclosed by C. A. Mead in “Operation of Tunnel-Emission Devices,” J. Appl. Phys., 32, 646 (1961). FIG. 26 shows a typical example of the construction of MIM type elements. The figure is a sectional view, in which reference numeral 3020 designates a substrate, numeral 3021 a lower electrode consisting of metal, numeral 3022 a thin insulating layer about 100 A thick and numeral 3023 an upper electrode about 80 to 300 A thick consisting of metal. In MIM type elements, electron emission is caused on the surface of the upper electrode 3023 by applying a proper voltage between the upper electrode 3023 and the lower electrode 3021.
The aforementioned cold cathode elements do not need a heater for heating their cathode since they allow electron emission to occur at a lower temperature than hot cathode elements. Accordingly, their structure can be simpler than that of hot cathode elements, which allows fine elements to be produced. Further, when multiple elements are densely arranged, problems such as melting substrate by heat and the like are unlikely to occur. In addition, unlike the hot cathode elements, which are slow at response because they operate only after heated with a heater, the cold cathode elements have the advantage of being quick at response.
Thus, a lot of studies have been conducted for the application of cold cathode elements.
A surface conduction type electron emission device, for example, has a particularly simple structure and is easy to produce compared with the other cold cathode elements, accordingly the application of this type elements is advantageous to forming multiple elements over a large area of the substrate. Therefore, methods have been studied to arrange and drive multiple elements on the substrate, as disclosed, for example, by the present applicants in Japanese Patent Application Laid-Open No. 64-31332.
As to the application of surface conduction type electron emission devices, the studies have been carried out of, for example, image producer such as an image display and an image recorder, charged beam sources and the like. For the application to an image display, the display using surface conduction type electron emission devices in combination with a fluorescent substance, which emits light when electron beam is applied, has been studied as disclosed by the present applicants in U.S. Pat. No. 5,066,883, Japanese Patent Application Laid-Open No. 2-257551 and Japanese Patent Application Laid-Open No. 4-28137. An image display using surface conduction type electron emission devices in combination with a fluorescent substance is expected to have properties superior to conventional ones using other methods. The above display may be superior to, for example, the liquid crystal display which has been in common use recently in that it does not need a backlight since it spontaneously emits light and in that it has a wide viewing angle.
A method for arranging and driving multiple FE type elements is disclosed, for example, by the present applicants in U.S. Pat. No. 4,904,895. The known examples of the application of FE type elements to an image display include, for example, a planar image display reported by R. Meyer et al. (refer to R. Meyer: “Recent Development on Micro-Tips Display at LETI,” Tech. Digest of 4th Int. Vacuum Microelectronics Conf., Nagahama, pp. 6-9 (1991)).
An example of the application of multiple MIM type elements in the arranged state to an image display is disclosed by the present applicants in Japanese Patent Application Laid-Open No. 3-55738.
Among the image producer using the electron emission devices described above, a planar image display which is thin depthwise has attracted considerable attention as a replacement of the image displays utilizing cathode-ray tubes, since it is space-saving and lightweight.
FIG. 27 is a perspective view of one example of the display panel constituting a planar image display, partially broken away to show the inside structure.
In the figure, reference numeral 3115 designates rear plate, numeral 3116 a side wall and numeral 3117 a face plate. And the rear plate 3115, the side wall 3116 and the face plate 3117 make up an outer enclosure (hermetic container) for keeping the inside of the panel cell vacuum. On the rear plate 3115 a substrate 3111 is fixed, while on the substrate 3111 N×M cold cathode elements are formed (wherein N, M are positive integers not lower than 2 and they are properly set according to the number of pixels to be displayed). The above N×M cold cathode elements 3112 are wired with M lines of row wiring 3113 and N lines of column wiring 3114 as shown in FIG. 27. The portion consisting of the substrate 3111, the cold cathode elements 3112, the row wiring 3113 and the column wiring 3114 is referred to as a multiple electron beam source. Between the row wiring 3113 and the column wiring 3114 an insulating layer (not shown in the figure) is formed at least at each portion where the row wiring intersects the column wiring. As a result, the row wiring 3113 and the column wiring 3114 can be kept electrically separated from each other.
On the underside of the face plate 3117, a fluorescent film 3118 is formed which consists of fluorescent substances of three primary colors: red (R), green (G) and blue (B) (not shown in the figure). Between adjacent fluorescent substances each of which is colored in any one of the above primary colors and constitutes the fluorescent film 3118, a black substance (not shown in the figure) is provided. And on the surface of the fluorescent film 3118 which faces the rear plate 3115, a metal back 3119 consisting of Al and etc. is formed.
Dx1 to Dxm, Dy1 to Dyn and Hv are electrical connection terminals having a hermetic structure for electrically connecting the above display panel with an electric circuit, which does not appear in the figure. Dx1 to Dxm, Dy1 to Dyn and Hv are electrically connected with the raw wiring 3113 of the multiple electron beam source, the column wiring 3114 of the multiple electron beam source and the metal back 3119, respectively.
The interior of the above hermetic container is kept at a vacuum of about 10−6 Torr (1.33×10−4 Pa). As the display area of the image display becomes larger, some means becomes necessary to prevent the rear plate 3115 and the face plate 3117 from undergoing deformation or fracture due to the difference in atmospheric pressure between the interior and the exterior of the hermetic container. The use of the method in which the rear plate 3115 and the face plate 3117 are made thicker not only increases weight of the image display, but causes distortion of images as well as parallax when viewing the display at an angle. Contrary to this, in FIG. 27 are provided structural supports (referred to as spacer or rib) 3120 made of a relatively thin glass plate for supporting atmospheric pressure. The spacing between the substrate 3111, which has a multiple electron beam source formed on it, and the face plate 3117, which has a fluorescent film 3118 formed on it, is usually kept submillimeter to several millimeters, and the interior of the hermetic container is kept at a high vacuum as described above.
When applying voltage to each cold cathode element 3112 in an image display with the display panel described above through the terminals, Dx1 to Dxm and Dy1 to Dyn, outside the container, electrons are emitted from each cold cathode element 3112. At the same time, a high voltage of several hundreds-volt to several-kilovolt is applied to the metal back 3119 through the terminal Hv outside the container to accelerate the emitted electrons above and force them to collide with the internal surface of the face plate 3117. This allows each colored fluorescent substance constituting the fluorescent film 3118 to be excited and emit light, as a result of which images are displayed.
The display is disclosed in U.S. Pat. No. 5,083,058 which uses glass containing, for example, ruthenium oxide for its struts and is the background art of the present invention.
The aforementioned display panel for image displays has, however, the following problems. First, the spacer 3120 may be charged when some of the electrons emitted from its vicinity hit it or when the ions emitted due to the action of the emitted electrons deposit to it. The orbit of the electrons emitted from the cold cathode element 3112 is deformed due to the charged spacer, so that the electrons reach the place other than the normal one, which leads to the distortion of the image in the vicinity of the spacer.
Second, there is a fear that a creeping discharge should occur along the surface of the spacer 3120 disposed between the multiple electron beam source and the face plate 3117, since a high voltage of several hundreds-volt or higher (that is, a high electric field of 1 kV/mm or higher) is applied therebetween to accelerate the electrons emitted from the cold cathode element 3112. An electric discharge is likely to be induced, particularly when the spacer is in the charged state as described above.
In order to solve this problem, there is proposed a method in U.S. Pat. No. 5,760,538 in which the electrical charge contained in spacers be neutralized by passing an infinitesimal current therethrough. In the above patent, an infinitesimal current is allowed to pass through the surface of the spacers by forming a highly resistant thin film as an antistatic film thereon. The antistatic film used in the above patent is a thin film of tin oxide, a mixed crystal thin film of tin oxide and indium oxide, or a metal thin film.
The use of the method in which electrical charge is neutralized with a highly resistant thin film sometimes leaves the problem of insufficient reduction of image distortion unsolved. The principal factor underlying this problem is considered to be the concentration of electrical charge in the vicinity of the junction portion due to the insufficient electrical junction between the spacers with a highly resistant thin film and the upper and lower substrates, that is, the face plate (hereinafter referred to as “FP”) and the rear plate (hereinafter referred to as “RP”). In order to solve this problem, there is proposed a method in which the end faces of the spacer facing FP and RP, respectively, are coated with the material whose resistivity is lower than a metal thin film or a highly resistive film within the range of about 100 to 1000 micron so as to ensure its electrical contact with the upper and lower substrates and control its electrification due to the incidence of the reflected electrons from the face plate, as disclosed in Japanese Patent Application Laid-Open No. 8-180821 and Japanese Patent Application Laid-Open No. 10-144203.
Even with such a means given to the highly resistive film and the means for controlling the orbit of emitted electrons, as well as with the formation of low resistive film portion for a better electrical contact as described below, electrification of the spacers cannot be sufficiently controlled depending on the other design parameters of the electron beam apparatus, such as materials and film thickness of its face plate, shape, and anode accelerating voltage, and there still exist problems of, for example, displacement of light emitting points and occurrence of an infinitesimal discharge in the vicinity of the spacers due to the insufficient control.
The cause of such electrification is not clarified in detail, it is, however, considered that the factors lie upon the following background.
Presumably, the cause of electrification of the spacers is such that there may exist some factors which effectively increase the capacitance and resistance of the spacers as described below, or the spacers are exposed to the reflected electrons from the cold cathode elements 3112 close thereto other than the most closest ones during their non-selective period and also exposed to the abnormal field emission from the field concentration region in the vicinity of the spacer-cathode junction. In addition, it is considered to be another cause of the electrification that control of the secondary emission coefficient on the surface of the spacers is not accounted for in design.
[Background 1] Restriction by the Relaxation Time Constant of a Highly Resistive Film on Spacers
The progress of electrification and relaxation in any region of the surface of a spacer can be considered as a time delay of the charged electric potential corresponding to the injection current by the application of a charged dielectric model.
FIG. 4 illustrates a model which represents the relaxation by capacitance resistant elements in the case of looking at upper and lower electrodes from a current injection region, when an effective injection current ic is supplied from a current source to an arbitrary position z on the surface of a spacer. In the figure, Va designates a voltage applied from a voltage source to an anode and ic an effective injection current supplied to the position at a height of zh (wherein h corresponds to the height of a spacer, 0<z<1). The effective injection current corresponds to the difference between a secondary current and a primary current. C1 and R1 designate values of capacitance and resistance, respectively, which specify the relaxation time constant between the injection region and the anode, while C2 and R2 values of capacitance and resistance, respectively, which specify the relaxation time constant between the injection region and the cathode. When the resistance and the capacitance distribute uniformly in the altitude direction, C1, C2, R1 and R2 are described using the resistance of the spacer R and the capacitance C by C/(1−z), R(1−z), C/z and Rz, respectively.
Since the principle of superposition should hold for the injection current in any position, the electric potential in the region of an arbitrary altitude on the spacer can be specified without losing generality if considering the electrification process in the following manner; first a high voltage Va from a voltage source is applied between the anode and the cathode, then the electronic current entering from the vacuum side to the position z in the aimed region is treated as an effective injection current Ic which is equivalent to the difference between the entered and emitted currents, and finally performing formularization with an equivalent circuit to which the effective injection current Ic as a current source is supplied, as shown in FIG. 4.
Now, in order to design a suitable spacer construction, formularization of a relaxation process will be performed taking a concrete example of the charged electric potential on the spacer having an insulating or highly resistive film formed on it and suitable for the electron beam emission apparatus of the present invention. For simplification, it is assumed that distribution of electric constant is uniform on the surface of the spacer. First, formularization is performed treating the rate of effective injection charge to the surface of the spacer as amount of current supplied from a current source and taking into account the energy distribution and incident angle distribution of incident electrons. The result is as follows:
amount of electronic current emitted from the electron emission device Ie
proportion of the incident electrons at an altitude of zh (0<z<1) βij 
secondary electron emission coefficient at an altitude of zh (0<z<1) δij 
provided that superscripts i, j correspond to incident energy and incident angle, respectively,
amount of primary electronic current in the position z IpIp=ΣΣIpij=ΣΣβij×Ie
amount of secondary electronic current in the position z IsIs=ΣΣδij×Ipij=ΣΣδij×βij×Ie
injection rate of the electrical charge in the position z IcIc=ΣΣ(δij−1)×Ipij=ΣΣ(δij−1)×βij×Ie
Finally, the rate of injection charge Ic can be described asIc=P×Ie  General Formula (2)
wherein P is described as P=ΣΣ(δij−1)×βij and is a coefficient independent of Ie, it is, however, assumed that in reality P changes as the progress of electrification.
Then, for the arrangement of the capacitance and resistance of the spacer film seen from the injection region, it is assumed for simplification that there exists neither resistance variation nor capacitance variation in the altitudinal direction of the spacer (this corresponds to the direction in which a high voltage is applied between anode and cathode). At this time, when the resistance and capacitance in the direction parallel to the surface of the spacer seen from anode/cathode are represented by R and C, the altitude of the spacer h, and the altitude of the injection region zh (0≦z≦1, on the anode side z=1), the electric constant existing above and below the injection region is specified for the position z. Further, since a voltage from the voltage source is applied between the anode and the cathode, an effective impedance Z is dealt with as 0. Thus, it is understood that the injected electrical charge undergoes relaxation through the parallel resistance and the parallel capacitance of each resistance and capacitance lying above and below the injection region. The resistance and the capacitance between the injection region in the position z and GND are described by z(1−z)R and C/z+C/(1−z), respectively, and response time constant τ of relaxation path corresponds to the product of the original resistance and capacitance of the spacer, that is, CR.
The electric potential in any position at this time is described as a function of time using the solution obtained by setting up a differential equation concerning a current for the entire close of the aforementioned equivalent circuit shown in FIG. 4.
When the time of starting electron emission is shown by t=0, provided that electron emission device is continuously driven, ΔV(t) which represents the progress of charged electric potential in the injection region is described by the following equation,ΔV(t)=z(1−z)Ric(1−exp(−t/τ))  General Formula (3)and it is clear that the progress of charged electric potential depends on the product of the resistance R and effective injection current Ic.
When plotting time in abscissa and the amount of the emission current from electron emission device and the time of emitting the charged electric potential electrons on the spacer in ordinate, setting quiescent time (that is, selective period, non-selective period) for t1 seconds, and repeating the drive of the element every t2 seconds, as shown in FIG. 5, the charged electric potential ΔV at the end of the first period (t1+t2 seconds) is described using the general formula (3) as follows:ΔV(t)=z(1−z)Ric(1−exp(−t1/τ)) exp(−t2/τ)  General Formula (4)And it is assumed that electrical charge is accumulated every time the elements close to the spacer are driven, provided that t2>>τ or t1<<τ does not hold. The relaxation process of electrification of the spacer is thus described.
On the other hand, the change in the position of a beam with the amount of electrons emitted during the selective period t1 (Duty dependency) is a problem for a display device, however such Duty dependency in the light emitting position can be dealt with as a change of ΔV shown by the general formula (3) corresponding to the amount of emitted electrons (the product of Ie and pulse width), accordingly both sides of the general formula (3) are differentiated by the amount of emitted electrons (the product of Ie and pulse width).
                                                                                                              ⅆ                    Δ                                    ⁢                                                                          ⁢                                      V                    ⁡                                          (                      t                      )                                                                                        ⅆ                                      (                                                                  I                        e                                            ⁢                                              t                        1                                                              )                                                              =                            ⁢                                                z                  ⁡                                      (                                          1                      -                      z                                        )                                                  ⁢                R                ⁢                                  {                                                                                    P                        ⁡                                                  (                                                      1                            -                                                          exp                              ⁡                                                              (                                                                                                      -                                                                          t                                      1                                                                                                        /                                  τ                                                                )                                                                                                              )                                                                    t1                                        +                                                                                                                                        ⁢                                                P                  ⁢                                                                          ⁢                                      exp                    ⁡                                          (                                                                        -                                                      t                            1                                                                          /                        τ                                            )                                                                      τ                            }                                                                          =                            ⁢                                                                    z                    ⁡                                          (                                              1                        -                        x                                            )                                                        C                                ⁢                                  P                                      t                    1                                                  ⁢                                  {                                      τ                    +                                                                  (                                                                              t                            1                                                    -                          τ                                                )                                            ⁢                                              exp                        ⁡                                                  (                                                                                    -                              t                                                        /                            τ                                                    )                                                                                                      }                                                                                                  General          ⁢                                          ⁢          Formula          ⁢                                          ⁢                      (            5            )                          ⁢                                      The general formula (5) is simplified by the driving conditions and the material constant. When the material is insulating or selective period is very short, CR=τ>>t1 holds, and the following formula is established.
                                                        ⅆ              Δ                        ⁢                                                  ⁢                          V              ⁡                              (                t                )                                                          ⅆ                          (                                                I                  e                                ⁢                                  t                  1                                            )                                      =                                            z              ⁡                              (                                  1                  -                  z                                )                                      ⁢            P                    C                                    General        ⁢                                  ⁢        Formula        ⁢                                  ⁢                  (          6          )                    When the material is low resistant or selective period is very long, CR=τ<<t1 holds, and the following formula is established.
                                                        ⅆ              Δ                        ⁢                                                  ⁢                          V              ⁡                              (                t                )                                                          ⅆ                          (                                                I                  e                                ⁢                                  t                  1                                            )                                      =                                            z              ⁡                              (                                  1                  -                  z                                )                                      ⁢            PR                                t            1                                              General        ⁢                                  ⁢        Formula        ⁢                                  ⁢                  (          7          )                    
Now parameters specifying Duty dependency in the light emitting position, that is, tone dependency during the selective period will be explained based on the above formularization.
In terms of the conditions under which an accelerating voltage between anode and cathode is maintained, preferably a spacer has some degree of insulating property or high resistance in the direction parallel to its surface. Accordingly, when taking into account Duty dependency of charged electric potential in any position, preferably the general formula (6) is applied. Thus, in order to control Duty dependency, dielectric constant or the section area of the spacer material needs to be enlarged. The controllable range of dielectric constant in material is, however, extremely limited compared with specific resistance, and as for film thickness, it is impossible to ensure an effective dimension for the reason related to processes. Thus, control of parameter P is required.
Further, in terms of the increase in effect of electrification relaxation during quiescent time, if electrons are injected into a spacer in a repetition period shorter than the time constant specified by resistance and capacitance, charges are accumulated, as described with respect to the above general formula (4). Even when the material is applied to the highly resistive film on the surface of the spacer whose relaxation time constant is smaller than the line non-selective period of electron emission device t2 second (≈selective period×the number of scanning lines), cumulative charge can be formed. Thus the design of relaxation time τ aiming at control of the resistance alone is considered to be insufficient for antistatic measures.
In any case, it is difficult to design suitable conditions under which electrification is restricted as long as control of resistance and capacitance alone is aimed at, for this purpose, the control of secondary electron emission coefficient is required
[Background 2] Generally secondary electron emission coefficient largely depends on the incident angle of incident electrons, and secondary electron emission coefficient δ doubles almost exponentially by enlarging the incident angle.
Generally, in cases where primary electrons enter the smooth surface as shown in FIG. 14, when the incident angle is represented by θ [degree] (−90<θ<90), incident energy by Ep [keV], the distance incident electrons penetrate into the film by d [Å], absorption coefficient of secondary by α [1/Å], the mean energy of primary electrons needed for the generation of secondary electrons in the film by ξ [eV] and the probability of secondary electrons escaping from the surface to vacuum by B, secondary electron emission coefficient is quantitatively described using parameters A, n describing the energy loss process of primary electrons in the film by the following general formula (0).
                    δ        =                              B                          4              ⁢              ξ                                ⁢                                    (                              An                                  α                  ′                                            )                                      1              n                                ⁢                                                    (                                                      α                    ′                                    ⁢                                      d                    p                                                  )                                                              1                  n                                -                1                                      ⁡                          [                              1                -                                                                  ⁢                                                                  ⁢                                                                  ⁢                                                      {                                          1                      +                                                                        (                                                                                    1                              γ                                                        -                            1                                                    )                                                ⁢                                                  α                          ′                                                ⁢                                                  d                          p                                                                                      }                                    ⁢                                      exp                    ⁡                                          (                                                                        -                                                      α                            ′                                                                          ⁢                                                  d                          p                                                                    )                                                                                  ]                                                          General        ⁢                                  ⁢        Formula        ⁢                                  ⁢                  (          0          )                    wherein parameters α, γ, dp are specified by the following relationship:
                                          α            ′                    =                      α            ⁢                                                  ⁢            cos            ⁢                                                  ⁢            θ                          ⁢                                  ⁢                              γ            =                          1              +                                                m                  1                                ×                                                      (                                                                  α                        ′                                            ⁢                                              d                        p                                                              )                                                        -                                          m                      2                                                                                                    ,                                          ⁢                                    m              1                        =            0.68273                    ,                                    m              2                        =            0.86212                          ⁢                                  ⁢                  dp          =                                    E              p              n                        An                                              General        ⁢                                  ⁢        Formula        ⁢                                  ⁢                  (                      0            ′                    )                    
The incident energy dependency of secondary electron emission energy shown by the above general formula (0) generally has an angle property with peaks, and in many cases, it has two incident energies with which the peak value of secondary electron emission coefficient δ exceeds 1 and the relation δ=1 is satisfied. In the incident energy between these two cross-point energies, secondary electron emission coefficient is positive, which means the generation of positive charge. Of the two cross-point energies, the smaller one is referred to as a first cross-point energy E1 and the bigger one a second cross-point energy E2.
Here, the incident angle dependency of secondary electron emission coefficient standardized in the general formula (0) for the vertical incidence of 0 degree, that is, θ=0 can be an index for evaluating the secondary electron emission multiplication effect at an angle.
This is shown below as a general formula (1),
                                          δ            θ                                δ            0                          =                                                                                        ⁢                                                                                          1                      -                                              {                                                  1                          -                                                                                                                    m                                0                                                            ⁢                              cos                              ⁢                                                                                                                          ⁢                              θ                                                                                      1                              +                                                                                                                                    (                                                                          m                                      1                                                                        )                                                                                                        -                                    1                                                                                                  ×                                                                                                      (                                                                                                                  m                                        0                                                                            ⁢                                      cos                                      ⁢                                                                                                                                                          ⁢                                      θ                                                                        )                                                                                                        m                                    2                                                                                                                                                                                                      }                                                                                                                                                                                                                            ⁢                                              exp                        ⁡                                                  (                                                                                    -                                                              m                                0                                                                                      ⁢                            cos                            ⁢                                                                                                                  ⁢                            θ                                                    )                                                                                                                                                        1              -                                                {                                      1                    -                                                                  m                        0                                                                    1                        +                                                                                                            (                                                              m                                1                                                            )                                                                                      -                              1                                                                                ×                                                      m                            0                                                          m                              2                                                                                                                                                            }                                ⁢                                  exp                  ⁡                                      (                                          -                                              m                        0                                                              )                                                                                ×                      1                          cos              ⁢                                                          ⁢              θ                                                          General        ⁢                                  ⁢        Formula        ⁢                                  ⁢                  (          1          )                    wherein parameters m1, m2 are constants having the following values:m1=0.68273, m2=0.86212
In the general formula (1), m0 is equal to and which is the product of the absorption coefficient of secondary electrons α and the penetration distance of primary electrons d, is a function of incident energy, and can be a positive real number. Hereinafter m0 is referred to as incident angle multiplication coefficient of secondary electron emission coefficient, because of its characteristics. In the above general formula (1), m0 shows a tendency to increase monotonously with the incident angle |θ| under arbitrary incident energy conditions, then rapidly increases where the incident angle becomes about 90 degrees. This is because the primary electrons enter the surface at an angle and the distribution of the secondary electron generating sites shifts near to the surface of the film. For this reason, the proportion of the electrons increases which are emitted into vacuum without recombining and therefore vanishing. This can be understood as an apparent reduction of the absorption coefficient of secondary electrons α to αcosθ. In the smooth thin film formed on the smooth surface of a spacer as a spacer material, for example, many antistatic films have an incident angle multiplication coefficient of secondary electron emission coefficient m0 larger than 10, provided that the incident energy having a positive secondary electron emission coefficient, which is larger than the first cross-point energy and smaller than the second cross-point energy, is 1 keV. This increases the positive electrification with the increase in the incident angle and is the big cause of the positive electrification of the spacer material. The enlarged incident angle multiplication effect of secondary electron emission coefficient is shown in FIG. 7 with black boxes.
[Background 3] The distribution of the incident angle to a spacer is large, in addition, the incident electrons entering the surface at a large incident angle are predominant.
There exist various routes for the electrons' incidence, they are, however, represented roughly by three particular routes. The first one is a direct incidence of the electrons emitted from electron emission devices. In this case, the incident angle is as large as about 80 to 86 degrees, though it depends on the degree of distortion in the electric field near the spacer and other designed values of the apparatus, and its incident mode is a large incident angle and high incident energy. Further, it has a feature such that, since the distance between the spacer and electron emission device close thereto is short, the amount of incident electrons is very large. The second one is an indirect incidence of the electrons reflected from a face plate to its surroundings. In this route, the distribution of the incident angle expands from 0 to large degrees, and the incident energy also has a distribution, but its range is smaller than that of the incident energy in the first route. The third one is re-incidence to the surface of the spacer of the incident electrons of the first and the second routes or the electrons emitted from field concentration points. This route is considered to occur because electrons are apt to re-enter the region in the locally positively charged state compared to other regions. In this case also, the incident angle has a distribution. Since a high electric field of about several kV/cm to several tens kV/cm is usually applied in the creeping direction as an accelerating voltage, the vertical incidence of electrons is modulated to an incidence at a large angle. Thus, incident electrons passing through any route have an incident angle distribution, and an effective charge injection is performed through the positive charge formed inside of a solid by the incident electrons entering at a large angle. Of the incident modes described above, the direct incident electrons of the first route is usually predominant over the positive charge in question, they are, however, dependent on the driving state and the design of electron emission device, and they can sometimes leave the problem unsolved of the reflected electrons from a face plate and the re-incidence of multiple scattered electrons described below.
[Background 4] Multiple Electron Emission on the Surface
The secondary electrons once emitted from the surface of a spacer have a relatively small initial energy of at most 50 eV. Although in space they receive energy from the electric field between the anode and cathode, since situations in which the spacer is charged positively often occur, there exist many electrons plunging into the positively charged region on the spacer as well as the electrons reaching the anode. These electrons are problematic because they accumulate the positive charge on the spacer cumulatively while repeating their incidence at a low incident energy and a large incident angle and emission alternately. Thus, control of the above multiple electron emission is the subject for study.
Now the above backgrounds will be abstracted. As apparent from Background 1, there are some cases where the film designed taking into account resistant value alone is not perfect since the range within which the dielectric constant and resistant value of the film can be selected is restricted, and in such a case it is important to restrict the amount of effective current injected into the film, or to restrict secondary electron emission coefficient.
As apparent from Backgrounds 2 and 3, in the design of the spacer's surface the reduction of incident angle dependency of secondary electron emission coefficient and the absolute value thereof is a subject, since electrification by the electrons with a large incident angle is predominant over the real electron emission devices. Further, Background 4 shows that it is important to reduce the cumulative emission phenomenon of electrons to control the cumulative positive accumulation of multiple scattered electrons. These are the subjects of the art of the present invention.
As described so far taking a spacer for example, there are some cases where there exists a member in a hermetic container within an electron emission apparatus which may be exposed to electrons, and the effect of the member due to its electrification is desired to be relaxed. The effects include, for example, variation of the position exposed to the electrons and occurrence of creeping discharge. The present patent application provides an invention which implements a construction enabling the relaxation of the above effects.